1. Field of Invention
This invention relates to the field of tissue engineering. More specifically, it relates to novel 3-dimensional hydrogel scaffolds which support the growth and maintenance of cells in culture, for use in simulated organ and tissue function, the study of cell-cell and cell-matrix interactions, development and testing of biological, pharmaceutical and biochemical compounds, as well as components of medical devices and apparatus.
2. Description of Prior Art
Tissue engineering seeks to repair, replace or restore tissue and/or organ function, typically by combining biomaterials and living cells.
There are many materials that are currently being used in tissue engineering applications. Natural polymers that have been used in this field include agarose, chitosan, hyaluronic acid, collagen, gelatin, and silk. Synthetic polymers have also been used for tissue engineering applications, including poly(L-lactic acid), poly(glycolic acid), poly(D,L-lactic acid-co-glycolic acid) and poly(caprolactone), poly(propylene fumarate), poly(orthoester), poly(anhydride), poly(ethylene glycol), poly(ethylene oxide), poly(methyl methacrylate) and poly(urethane).
These natural and synthetic polymers when used in tissue related applications, are often referred to as biomaterials. Biomaterials are used in a variety of related applications, such as in the orthopedic field for joint and cartilage replacement and repair. Biomaterials can be used in applications such as bone cement, artificial ligaments and tendons, vascular grafts, heart valves, stents, and blood substitutes. Further uses for biomaterials can be artificial lenses, degradable sutures, dental implants, burn dressings, artificial skin, and as a drug or biologic delivery device.
The inherent properties of the biomaterial used in a tissue engineered construct emerge from the local response of the cells to their 3-dimensional microenvironment. It is therefore of great importance to re-create biochemical and structural components of the in vivo cellular microenvironments when designing implantable tissue constructs. This microenvironment can be simulated by patterning of the matrix in which the cells are grown in or on, or by patterning the cells within the matrix. For example, scaffold texture can alter cell migration, ingrowth, vascularization, and host integration. Microscale scaffold architecture can also modify the cellular responses such as growth and differentiation as has been shown on three-dimensional polymer meshes (e.g. U.S. Pat. No. 5,443,950).
Methods to prepare scaffolds with microscale structure that are more amenable to use with biodegradable polymers such as poly-DL-lactide-co-glycolide (PLGA) have also been developed. Material microstructure was first controlled by process parameters such as the choice of solvent in phase separation, doping with particulate leachants, gas foaming, woven fibers, and controlled ice crystal formation and subsequent freeze-drying to create pores; however, these scaffolds lack a well-defined organization that is found in most tissues in vivo (i.e. pores are randomly distributed rather than oriented and organized in functional units). Similarly, microtubular scaffolds, 3-dimensional micropatterned scaffolds using UV polymerization, can also produce scaffolds with arbitrary architectures.
Hydrogels are becoming an increasingly popular material for tissue engineering because their high water content and mechanical properties resemble those of tissues of the body. In addition, many hydrogels can be formed in the presence of cells by photopolymerization, which allows homogeneous suspensions of cells throughout the gel. Poly (ethylene glycol) (PEG)-based hydrogels are of particular interest because of their biocompatibility, hydrophilicity and the ability to be customized by changing the chain length or chemically adding biological molecules. PEG based hydrogels have been used to homogeneously immobilize various cell types including chondrocytes, vascular smooth muscle cells, and fibroblasts that can attach, grow and produce matrix.
Generally, it is understood in the art that the synthetic biomaterial should provide a matrix for the biological tissue to fill and then slowly degrade and be absorbed by the tissue. Few synthetic degradable biopolymers have been studied for use in tissue engineering applications. Generally, the degradable synthetic polymers developed were intended for use in plastics that are biodegradable in the environment. Most of the chemistry of degradable biopolymers is based on an ester polymer backbone. Such materials include poly(L-lactic acid) (PLLA), poly(glycolic acid) (PGA), poly(D,L-lactic acid-glycolic acid)(PLGA), poly(propylene glycol-co-fumaric acid) (PPF) and poly(caprolactone) (PCL). Polymer materials like these degrade when in the presence of water, as a result of the water molecule reacting with the ester linkage of the polymer backbone.
PLLA is attractive as a biomaterial because there exists outstanding possibilities for the modification of its properties via copolymerization and stereocopolymerization and compounding. PLLA has a hydrophobic nature, and as such allows for protein absorption and cell adhesion, making it a suitable biomaterial for tissue engineering purposes. PGA has been shown to exhibit increased cell attachment properties when compared to PLLA. PLGA, the copolymer of PLLA and PGA, has mechanical properties and a degradation rate that can be controlled by adjusting the ratio of PLLA:PGA. PLLA, PGA, and PLGA are Food and Drug Administration-approved and are currently being used as biomaterials for tissue engineering applications, as resorbable sutures, as bone plates and screws, and in drug delivery devices. PPF is covalently crosslinked by means of its double bond, leading to increased mechanical properties. Additionally, PPF is attractive because its crosslinking is photoinitiated and can therefore be cured in situ. PCL is favorable as a biomaterial because its properties can be tailored by copolymerization with collagen, PGA, and poly(ethylene oxide). It is important, however, that all of the chains of a synthesized polymer be identical because chain length is a determining factor of the degradation properties of a biomaterial.
Another property of synthetic polymers is that their mechanical properties can be compromised as they degrade. Aside from mechanical degradation, the major disadvantage to the use of the prior art biomaterials in biological tissue applications is that their degradation products are acidic. As the scaffold degrades, the local pH of the tissue becomes quite acidic. The acidity initiates an immune response from the recipient that leads to increased inflammation at the site. The corresponding inflammation results in further premature degradation of the biopolymer scaffold.
There have been reports of concern raised about the biocompatibility of these materials have been raised when PLA and PGA produced toxic solutions as a result of acidic degradation in situ.
As such, there exists in the art a need for development of a biocompatible, water-soluble biomaterial, which does not produce acidic or toxic byproducts as a result of biodegradation or absorption in the host tissues.